1. Field of the Invention
The present invention relates to a disk cartridge and in particular to an improved integrally-molded plastic shutter of the disk cartridge.
2. DESCRIPTION OF THE RELATED ART
A prior-art disk cartridge shutter is made for example of an about 0.2 mm thick U-shaped stainless steel plate. Since, however, the stainless steel plate is disadvantageous because it is difficult to attain high dimensional accuracy in the stainless steel plate, the cartridge case of plastic is likely to be abraded by the edge of the steel plate, and the shutter is easily detached from the cartridge case upon impact when the disk cartridge is dropped. Under these circumstances, research and development of the integrally molded plastic shutter has been carried out.
The plastic shutter must be molded in the form of a thin-walled product because the restricted thickness of the disk cartridge restricts the thickness of the shutter therefor. When the thin shutter is injection-molded a molten plastic flows at a high speed in the injection molds, and therefore the molten plastic is subjected to a shear stress and molecules of molten plastic are likely to be stretched in a flow direction of the molten plastic and solidified to have a molecular orientation. Since the stretched plastic molecules are subject to a residual stress causing them to be restored to their initial or unstretched condition, the conventional plastic shutters thus produced entail a problem in that they experience a dimensional change and a deformation with passage of time when they are left in a high-temperature environment. In addition, since the molecular orientation causes the plastic molecules to be less entangled in the direction transverse to the flow direction of the molten plastic, the resulting thinly molded plastic shutter may be readily split or broken along the flow direction of the molten plastic.